"I shouldn't say this, but there is an American maxim that when a politician is in trouble, get him a dog" he added.

Following his appearance on LBC, Watson said there were a number of people who say that they would vote Labour if there was a general election tomorrow but are currently considering voting Conservative because they either like Boris Johnson or don’t like Ken.

"I was trying to say that it is important for our campaign to convince these people that even if they didn’t like Ken they should consider voting Labour because David Cameron would claim victory if Boris wins," he wrote on his blog.

"From what our campaigners on the ground say, the last thing this group want to do is give an endorsement to David Cameron."

He added: "There is a great picture of Ken and his beautiful labrador dog in today’s Guardian. That prompted me to crack the Harry Truman maxim about politicians and dogs."